The field of this invention relates to a method and apparatus for calibrating an envelope tracking system, and in particular to a method and apparatus for calibrating an envelope tracking system of a radio frequency (RF) transmitter of a wireless communication unit using power-aware power amplifier gain mapping.
Continuing pressure on the limited spectrum available for radio communication systems is forcing the development of spectrally-efficient linear modulation schemes. Since the envelopes of a number of these linear modulation schemes fluctuate, these result in the average power delivered to an antenna being significantly lower than a maximum available power, leading to poor efficiency use of the power amplifier. Specifically, in this field, there has been a significant amount of research effort in developing high-efficiency topologies capable of providing high performances in the ‘back-off’ (linear) region of the power amplifier.
Linear modulation schemes require linear amplification of the modulated signal, in order to minimise undesired out-of-band emissions resulting from spectral re-growth. However, the active devices used within a typical RF amplifying device are inherently non-linear by nature. Only when a small portion of the consumed DC power is transformed into RF power, can the transfer function of the amplifying device be approximated by a straight line, i.e. as in an ideal linear amplifier case. This mode of operation provides a low efficiency of DC-to-RF power conversion, which is unacceptable for portable (subscriber) wireless communication units. Furthermore, low efficiency is also recognised as being problematic for base stations in wireless communication systems.
Additionally, an emphasis in portable (subscriber) equipment is to increase battery life. To achieve both linearity and efficiency, so called linearization techniques are used to improve the linearity of the more efficient amplifier classes, for example class ‘AB’, ‘B’ or ‘C’ amplifiers. A number and variety of linearizing techniques exist, which are often used in designing linear transmitters, such as Cartesian Feedback, Feed-forward, and Adaptive Pre-distortion.
Voltages at the output of the linear, e.g. Class AB, amplifier are typically set by the requirements of the final RF power amplifier (PA) device. Generally, the minimum voltage of the PA is significantly larger than that required by the output device of the Class AB amplifier. Hence, they are not the most efficient of amplification techniques. The efficiency of the transmitter (primarily the PA) is determined by the voltage across the output components/devices, as well as any excess voltage across any pull-down device components, due to the minimum supply voltage (Vmin) requirement of the PA.
Current communication systems and communication units need to support signals with wide bandwidth and high peak-to-average power ratio (PAPR). In order to increase the bit rate used in transmit uplink communication channels, larger constellation modulation schemes, with an amplitude modulation (AM) component, are being investigated and, indeed, becoming required. These modulation schemes, such as sixteen-state quadrature amplitude modulation (16-QAM), require linear PAs and are associated with high ‘crest’ factors (i.e. a degree of fluctuation) of the modulation envelope waveform. This is in contrast to the previously often-used constant envelope modulation schemes and can result in significant reduction in power efficiency and linearity.
A conventional power amplifier (PA) with a fixed supply voltage (VPA) should be operated at the so-called ‘back-off’ region. However, the PA supply voltage is also typically set high enough to linearly amplify high PAPR signals. Thus, a large portion of the supplied energy is dissipated as heat when the instantaneous power is lower than the peak power, thereby making the efficiency of the power stages of the transmitter when operating at lower power to be much less than when operating at peak output power.
To help overcome such efficiency and linearity issues a number of solutions have been proposed. One of the more popular efficiency improvement techniques is known as envelope tracking (ET) and relates to modulating the PA supply voltage to match (track) the envelope of the radio frequency waveform being transmitted by the RF PA. With envelope tracking (ET), the instantaneous PA supply voltage of the wireless transmitter is caused to approximately track the instantaneous envelope (ENV) (e.g. amplitude) of the transmitted quadrature (I/Q) radio frequency signals. Thus, since the power dissipation in the PA is proportional to the difference between its supply voltage and output voltage, envelope tracking enables an increase in PA efficiency, reduces heat dissipation, improves linearity and increases maximum linear output power, whilst allowing the PA to produce the intended RF output. Since the overall efficiency of an ET systems is proportional to the PA efficiency, optimized design of envelope (ENV) shaping function is very important.
FIG. 1 illustrates a graphical representation 100 of the aforementioned two alternative PA supply voltage techniques; a first technique that provides a fixed supply voltage 105 to a PA, and a second technique whereby the PA supply voltage is modulated to track the RF envelope waveform 115. In the fixed supply voltage case, excess PA supply voltage headroom 110 is used (and thereby wasted), irrespective of the nature of the modulated RF waveform being amplified.
However, for example in the PA supply voltage tracking of the RF modulated envelope case 115, excess PA supply voltage headroom can be reduced 120 by modulating the RF PA supply, thereby enabling the PA supply to accurately track the instant RF envelope.
The mapping function between envelope and the variable PA supply voltage (Vpa) is critical for optimum performance (e.g. efficiency, gain, and adjacent channel power (ACP)). FIG. 1 also shows graphically 150 a plot of PA gain 155 versus output power (Pout) 160 for various fixed supply voltages with a maximum gain 165 of 28 dB for each fixed supply voltage reduced through gain compression 170 to an operating gain 175 of 25 dB.
Envelope-tracking can be combined with digital pre-distortion (DPD) on the RF signal to improve ACP robustness. The inventors of the present invention have also recognized and appreciated that envelope tracking with equal-gain mapping has several limitations, for example a lower equal-gain target provides better PA efficiency (but the efficiency improvement is limited due to the limited maximum output power capability of the RF transceiver) and that DPD AM/AM compensation may be insufficient to overcome high PA gain compression.
US 2012/0105150 A1, titled ‘joint optimization of supply and bias modulation’, describes an ET LUT that is populated to linearize a PA based solely on determined PA amplitude modulated-to-amplitude modulated (AM/AM) characteristics. The described technique uses an envelope shaping technique to provide maximum linearity whilst using fixed gain, irrespective of the required output power and supply voltage. The inventors have identified that lower equal-gain target provides good PA efficiency, but the efficiency improvement is limited due to the limited maximum output power capability of the RF transceiver.
A paper by D. Kim et al., titled ‘optimization for envelope shaped operation of envelope tracking power amplifier’, published in IEEE TMTT, vol. 59, no. 7, July 2011, pp. 1787-1795, describes a sweet-spot tracking technique that populates an ET LUT in order to adjust the supply voltage to follow the sweet spot points at each output power level to maintain good PA linearity.
Thus, there is a need for an efficient and cost effective solution to the problem of ET system calibration. In particular, it would be advantageous to provide an increased efficiency-improvement range whilst operating under current RF transceiver output power constraints.